1. Field of the Invention
The present invention relates to a magnetic garnet material for use in optical communication systems, a Faraday rotator, an optical device, a bismuth-substituted rare earth-iron-garnet single-crystal film and a method for producing the same and a crucible for producing the same, especially to a technique for obtaining a Faraday rotator that has the advantages of reduced insertion loss and improved magnetic properties.
2. Description of the Related Art
Optical communication is now being much popularized instead of electric communication of small transmission capacity. The reason is essentially because optical communication enables high-speed large-capacity transmission, it is favorable for long-distance transmission not requiring too many relays, and it is not influenced by electromagnetic noise, as will be described hereinunder.
Light falls in the same concept as that of radio waves used in TV and radio broadcasting and in radio communication, in that they are electromagnetic waves. However, the number of frequency of electromagnetic waves that are used in optical communication is about 200 THz and is about 20000 times that of radio waves (about 10 GHz) used in satellite broadcasting. The higher frequency means shorter wavelength, and enables high-speed transmission of more signals. The wavelength (center wavelength) of electromagnetic waves that are used in optical communication is 1.31 μm and 1.55 μm.
As well known, optical fibers that are used in optical communication have a two-layered structure of glass in which the refractive index of one layer differs from that of the other layer. The light that runs through the center core of such optical fibers is repeatedly reflected inside the core, and therefore optical fibers can accurately transmit signals through them even though they are bent. In addition, since high-purity quartz glass of high transparency is used for optical fibers, the data attenuation in optical communication through them is only about 0.2 dB per km. Accordingly, optical fibers enable data transmission even to a distance of about 100 km with no amplifier therein, and as compared with that in electric communication, the number of relays in optical communication may be reduced.
Electric communication shall face a problem of EMI (electromagnetic interference), but optical communication through optical fibers is not influenced by electromagnetic induction noise. Therefore, optical communication enables information transmission of extremely high quality.
In current optical communication systems, electric signals are converted into optical signals by LD (laser diode) in light transmitter. The optical signals are transmitted by optical fibers and then converted into electric signals by PD (photodiode) in light receiver. In that manner, the indispensable elements in optical communication systems are LD, PD, optical fibers and optical connectors. Apart from relatively low-speed short-distance communication systems, high-speed long-distance communication systems require optical transmission devices such as optical amplifier and optical divider, and also other optical devices to be combined with them, such as optical isolator, optical coupler, optical branching filter, optical switch, optical modulator and optical attenuator, in addition to the elements mentioned above.
In high-speed long-distance transmission or multi-branch optical communication systems, the element that is especially important is optical isolator. In current optical communication systems, optical isolators are used in LD modules of optical transmitters and in relays. The role of optical isolator is to transmit light only in one direction and to interrupt the light that has reflected in its running course to turn back. Faraday effect, a type of magneto-optical effect is applied to optical isolator. Faraday effect indicates a phenomenon of such that the plane of polarization of light is rotated after having passed through a transparent medium that is in a magnetic field applied thereto. The phenomenon of light of which the plane of polarization is rotated is referred to as rotary polarization. Magneto-optical rotation (Faraday rotation) to be caused by Faraday effect differs from ordinary optical rotation (natural optical rotation) in that, even when the light traveling direction is reversed, its rotary direction does not still change. An optical element that takes advantage of the phenomenon of rotary polarization owing to Faraday effect is referred to as a Faraday rotator.
Faraday rotator has some influence on the performance of optical isolator that comprises it. Accordingly, the properties of the materials to constitute Faraday rotators are important for obtaining optical isolators of high performance. The matters of importance in selecting the materials to constitute Faraday rotators are that the selected materials enable a large Faraday rotation angle at the wavelength of light for service (1.31 μm and 1.55 μm for optical fibers) and their transparency is high. At first, YIG (yttrium-iron-garnet; Y3Fe5O12) was used for the material that satisfies the requirements. However, the problems with YIG are that its mass-scale productivity is low and its size reduction is difficult. Meanwhile it has been found that, when the rare earth site of garnet-type crystals is substituted with Bi (bismuth), then the Faraday rotation performance of the resulting crystals is drastically improved. After that, Bi-substituted rare earth-iron-garnet single-crystal film (hereinafter this will be simply referred to as “garnet single-crystal film”) has been used for Faraday rotators.
Bi-substituted rare earth-iron-garnet single-crystal film is formed in a mode of liquid phase epitaxial growth (LPE). In a process of LPE, bismuth oxide, and rare earth, iron and garnet oxides (starting components) that include, for example, ferric oxide and rare earth oxides are formed into a starting material composition along with a flux component that contains lead oxide and boron oxide, and this is put into a Pt crucible. Next, the crucible is heated at a predetermined temperature in which the starting components are thereby melted into a melt. Next, the melt is cooled to be in a supercooled condition, and it is contacted with an LPE substrate while the substrate is rotated, whereby the intended garnet single-crystal film epitaxially grows on the LPE substrate.
One problem which has heretofore been pointed out with the LPE process of forming such a garnet single-crystal film is that the crucible used is corroded. It is well known that Pt is highly resistant to corrosion, but its corrosion resistance to lead oxide that forms the flux component and to the bismuth oxide-containing melt is unsatisfactory. Accordingly, even Pt crucibles are significantly corroded at the area around the face of the melt therein when the level of the melt is kept constant.
For preventing crucibles from being corroded by the melt therein, JP-A 9-175898 (hereinafter referred to as “Reference 1”) describes a method of moving the level of the melt in a crucible to thereby move the region of the crucible that may be corroded by the melt, and this is for controlling the amount of dissolution of crucibles. JP-A 11-322496 (hereinafter referred to as “Reference 2”) describes a method of providing, inside a Pt crucible, a Pt corrosion inhibitor separable from the Pt crucible. The methods described in Reference 1 and Reference 2 will be good since they are effective for reducing the frequency of exchanging or recasting Pt crucibles.
However, the methods described in Reference 1 and Reference 2 could not still reduce the amount of Pt to be released from corroded crucibles to contaminate the melt in the crucibles. Pt taken in the garnet single-crystal film increases the light absorption of the film. This is because the element Pt to be a tetravalent cation is taken in the garnet single-crystal film and the film therefore loses its charge balance, and the light absorption characteristics of the film are thereby worsened.
For preventing the light absorption characteristics of such garnet single-crystal film from being worsened, some methods have heretofore been investigated. For example, the film is subjected to suitable heat treatment, or a suitable amount of an element to be a divalent cation (e.g., Ca) or an element to be a tetravalent cation (e.g., Ge) is added thereto so as to keep the charge balance of the film. However, even the heat treatment or the addition of the element to be a divalent or tetravalent cation could not always satisfactorily restore the light absorption characteristics of the film.